Abstract X-ray diffraction (XRD), field emission/focusing ion beam scanning electron microscopy (FE-/FIB-SEM), nitrogen gas adsorption (N2GA), and geochemical analyses were performed on the Lower Longmaxi shale in the southeastern Sichuan Basin and its adjacent areas to reveal the controls of shale lithofacies and organic matter (OM) on pore development. The results showed that the Lower Longmaxi shale primarily comprises three types of lithofacies based on shale compositions and sedimentary environment: OM-rich siliceous shale (ORSS), OM-moderate mixed shale (OMMS) and OM-lean argillaceous shale (OLAS). ORSS comprises high contents of OM and siliceous minerals (quartz and feldspar), deposited in a deep-water shelf and early semi-deep-water shelf environment, while OLAS consists of high clay mineral and low OM contents, primarily formed in a shallow-water shelf environment. OMMS witness a transition from ORSS to OLAS in sedimentary environment and shale components. OM hosts a higher volume of pores per unit weight compared to inorganic minerals, corresponding to 0.1878 ml/g and 0.00396 ml/g, respectively. OM-hosted pores are dominant in ORSS, constituting 72%, whereas OLAS mainly consists of mineral-hosted pores, comprising 63.2%. OM-hosted and mineral-hosted pores in OMMS exist in similar proportions, namely, 56.4% and 43.6%, respectively. Differentiated diagenetic paths resulting from sedimentary environment are responsible for the differences in pore development under various shale lithofacies. The thermal maturity, macerals, and abundance of OM all play a significant role in the morphology and development of OM-hosted pores. OM-hosted pores are primarily founded in migrated OM (mainly sapropelic type), instead of structured OM (mainly humic type), in the form of secondary pores that are derived from dry gas-generation stage. Furthermore, OM-hosted pore development in shale gas reservoirs is slowed or even reduced because of the weakening of shale's resistance to compaction resulting from excessively high OM content.

中文翻译：

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川东南及邻区龙马溪下页岩孔隙发育——岩相识别与有机质洞察

摘要 利用 X 射线衍射 (XRD)、场发射/聚焦离子束扫描电镜 (FE-/FIB-SEM)、氮气吸附 (N2GA) 和地球化学分析对四川盆地东南部龙马溪下页岩和其邻近区域揭示页岩岩相和有机质 (OM) 对孔隙发育的控制。结果表明，根据页岩成分和沉积环境，下龙马溪页岩主要包括三种类型的岩相：富OM硅质页岩（ORSS）、OM-中混页岩（OMMS）和OM-贫泥质页岩（OLAS）。ORSS 由高含量的 OM 和硅质矿物（石英和长石）组成，沉积在深水陆架和早期半深水陆架环境中，而 OLAS 由高粘土矿物和低 OM 含量组成，主要形成于浅水陆架环境。OMMS 见证了在沉积环境和页岩组分中从 ORSS 到 OLAS 的转变。与无机矿物相比，OM 每单位重量的孔隙体积更大，分别对应于 0.1878 ml/g 和 0.00396 ml/g。ORSS 中以 OM 为主的孔隙占 72%，而 OLAS 以矿物为主的孔隙占 63.2%。OMMS 中的 OM 赋存和矿物赋存孔隙以相似的比例存在，分别为 56.4% 和 43.6%。不同页岩岩相下孔隙发育差异是由沉积环境形成的差异化成岩路径造成的。OM 的热成熟度、微晶成分和丰度都对 OM 承载的孔隙的形态和发育起着重要作用。OM 赋存孔隙主要形成于迁移的 OM（主要是腐泥型），而不是结构化的 OM（主要是腐殖质类型），其形式为干气生成阶段衍生的次生孔隙。此外，由于 OM 含量过高导致页岩抗压实能力减弱，导致页岩气藏中 OM 赋存的孔隙发育减缓甚至减少。